1. Field of the Invention
The invention relates to a device of the mirror type in the range of X-UV rays, comprising the periodic stack, on a support, of a so-called system of superimposed layers, which system includes a lower layer of a so-called first heavy element reflecting at the wavelengths of application of the mirror, and an upper layer of a light element referred to as a spacer which has low optical absorption at these wavelengths.
The invention is applied, for example, in the construction of spectroscopes, microscopes or telescopes using X-rays, and any apparatus requiring the construction of X-ray mirrors exhibiting coefficients of reflection which are as great as possible, or again in the construction of monochromators, which require mirrors which furthermore exhibit a good selectivity.
2. Description of the Prior Art
Mirrors intended for the X-ray range are known from the publication entitled "Interface analysis of sputtered W/C, Rh/C and Ni/C multilayers for soft X-ray applications" by P. BOHER, Ph. HOUDY and Cl. SCHILLER in "Thin Solid Films, 1989, p. 175".
This publication teaches that in order to construct mirrors in the X-ray range, it is necessary to form periodic stacks of systems of two layers, one of which consists of a light element which has low absorption and which is generally designated under the name of spacer, and the other layer of which consists of a reflecting heavy element.
The cited publication teaches that these two layers may be selected from among the pairs W-C, Rh-C, and Ni-C, in which carbon is the spacer element, and tungsten, rhodium or nickel is the reflecting heavy element. The pair W-C is that which, at the present time, is best known.
The thickness of the system of two layers, or 1 period thickness, is of the order of the operating wavelength, that is to say of a few nm in the X-ray range.
This publication also teaches that the two parameters which have the greatest influence on the reflectivity are:
the roughness of the interfaces of the layers,
the reproducibility of the thicknesses.
It is stated that the reproducibility of the thicknesses is essentially an apparatus problem, while the roughness of the interfaces is dependent upon the intrinsic properties of the materials selected to construct the layers.
Thus, according to this publication, the W-C interface exhibits a roughness of the order of 0.4 nm, while the C-W interface is almost perfect.
The case of the pair Rh-C is more complex. It appears that the rhodium contaminates the carbon layer by diffusion, while the carbon reduces the intrinsic roughness of the Rh layer. The result of this is that the C-Rh interface exhibits a thickness of the order of 0.5 nm, and is followed by a pure rhodium layer.
The case of the pair Ni-C is the most complex. On the one hand, an intense interdiffusion of the nickel into the carbon appears, and on the other hand the C-Ni interface likewise appears to be controlled by the interdiffusion of the carbon into the nickel, even after a deposit thickness of 2 nm.
It is also known to construct mirrors for X-rays from the publication entitled "The structure of ultra-thin C/W and Si/W multilayers for high performance in soft-X-ray optics" by P. RUTERANA et al. in "J. Appl. Phys. 65 (10) May 15, 1989" p. 3907-3913.
According to this document, mirrors for X radiation are constructed by means of multilayers formed from the pair silicon-tungsten. In this pair, the silicon is the spacer element and the tungsten is the reflecting heavy element. It is also taught that in order to improve the reflectivity it is necessary to minimize the interface roughness. The materials of the pair are selected for this purpose by utilizing the coalescence as a criterion. A low coalescence thickness promotes the production of a minimum interface roughness.
Now, it is known to the person skilled in the art that the wavelength of application of the mirror is linked to the choice of the spacer element--reflecting element pair; in other words, for a given application and a defined source of X-rays, the selectivity and the reflectivity of the mirror are determined essentially by the selection of the materials of the two-layer system.
However, on the other hand, as demonstrated by these cited documents, the materials which form the best candidates for the construction of a mirror at a given wavelength, for a selected application, may in reality not be appropriate for a satisfactory industrial application, by reason of the fact that they exhibit interface roughnesses or defects linked to the interdiffusion.